Porous membranes, or filters, have been developed for use in a wide variety of applications. For example, membranes can be used to separate isotopes, to filter liquids, to purify gas streams and for other separation processes.
Different types of porous membranes are described in the prior art. Membranes can have a variety of shapes and have been fabricated from different materials. For example, Canadian Pat. No. 1173308 by Clement et al. discloses a process for making a tubular microporous filter element. The process includes forming a layer of fine particles on a porous tubular substrate and applying hydrostatic pressure to decrease the pore radii. It is disclosed that the filter element is useful for separating isotopes or very fine particles.
U.S. Pat. No. 4,738,874 by Berardo et al. discloses a method for producing mineral membranes. Particles selected from mineral compounds such as simple or mixed metal oxides are dispersed on a substrate, such as an alumina substrate, from a liquid suspension. The composite membrane is then annealed at an elevated temperature to form a rigid membrane.
For many applications, a rigid and inflexible membrane is undesirable. It has therefore been proposed to form membranes having a ductile metallic substrate. For example, U.S. Pat. No. 4,888,114 by Gaddis et al. discloses a process for forming a filter having a metallic base. Metal oxide particles (e.g. TiO.sub.2) having a size of from 0.2 to 1.0 micrometer are drawn into a porous metal substrate (e.g. stainless steel) having a pore size of from about 0.5 micrometers to about 10 micrometers and excess metal oxide particles are then removed from the surface of the substrate. The metal oxide particles within the metal substrate are then sintered to form a filter element.
U.S. Pat. No. 4,935,139 by Davidson et al. discloses a process for fabricating a composite membrane. A metallic support having an average pore size of from 1 to 10 micrometers is covered with a porous film of sintered non-metallic particles having two average particle size distributions, one having particles from 0.5 .mu.m to 50 .mu.m and the other having particles from 4 nm to 1 .mu.m. The smaller particles act as a sintering aid. Sintering of the composite places the film in biaxial compression due to thermal expansion mismatch of the film and the metallic substrate.
U.S. Pat. No. 4,613,369 by Koehler discloses a method for making a porous metal filter. A stabilized suspension of dispersed metal particles is applied to a porous metal support, such as a wire mesh screen, to infiltrate the openings in the porous metal support. Excess particles are removed from the surface of the support with a doctor blade. The support is then heated to dry the stabilized suspension of metal particles and is compressed between rollers to decrease the pore size and improve the sintering characteristics. The support is then sintered to fuse the individual particles of the metal particulates to the metal support and to each other. The metal particulates have a particle size in the range from about 1 .mu.m to about 200 .mu.m.